You'll have to increase the limit to more than 10MB for an 8MB file, when a file is uploaded and converted to mime format it can occupy 30% more than the original file size. You need to do the following:

Bill knows what he's talking about-though I think he meant to flip the values so that 'MtaMax' is larger than FileMax'

EDIT: ok, so this is on top of zimbraFileUploadMaxSize 30% > than your 10MB limit being asked about in your posts.

Also as a general rule, I set zimbraMtaMaxMessageSize 10% larger than the zimbraFileUploadMaxSize.

Say both are set to 10MB (and you tell everyone they can send 10MB attachments):
An email with a 9MB attachment and 100KB message body text. The attachment may be less than the 10MB zimbraFileUploadMaxSize, and so it'll get accepted by the file upload system. But when it's converted into a MIME message it ends up base64-encoded. Then add 100K of regular body text, and the resulting message (say 10.3MB or so) is then handed off to postfix, which checks zimbraMtaMaxMessageSize and promptly rejects the message if it's over the limit of 10MB.

Of course set zimbraMtaMaxMessageSize even larger if you want to allow emails with multiple large attachments.

Also, there's a spamassassin setting to not scan/spend cpu resources on really large messages that are over x value in size - adjust accordingly if your willing to risk a little less protection in tradeoff for quicker large file delivery.

Also, there's a spamassassin setting to not scan/spend cpu resources on really large messages that are over x value in size - adjust accordingly if your willing to risk a little less protection in tradeoff for quicker large file delivery.